This invention relates to polycarbonate technology, and more particularly to the development of optical quality polycarbonates.
Optical data-recording media, including optical disks as exemplified by compact audio disks and CD-ROM disks used in computers, have become a major means of storing data and making it available. The data on an optical disk are read by a plane polarized laser beam and a polarization-sensitive detection scheme. For this reason, it is necessary to minimize polarization-dependent effects on these laser beams upon passage through the disk.
By far the most common polymers employed in optical disks are polycarbonates. They are peculiarly suited for this purpose by reason of their transparency and favorable physical properties.
In the further development of optical disks, particularly read-write disks and disks capable of storing larger amounts of data, various physical factors become important. One such factor, which is closely related to the storage capacity of the disk, is birefringence, i.e., the difference between indices of refraction for light polarized in perpendicular directions. Birefringence leads to phase retardation between different polarization components of the laser beam, thereby reducing readability.
Birefringence results from several sources, including the chemical nature of the raw material from which the disk is fabricated, the degree of molecular orientation therein and thermal stresses in a fabricated plastic optical disk. The observed birefringences of a disk are therefore determined by the molecular structure, which determines the intrinsic birefringence, and the processing conditions, which can create thermal stresses and orientation of the polymer chains.
It is known that bisphenol A polycarbonates, which are the principal ones currently being produced, are characterized by very high positive intrinsic birefringence, "bisphenol A" being the common name of 2,2-bis(4-hydroxyphenyl)propane. It is also known that homopolycarbonates comprising units derived from spiro(bis)indanes, especially 6,6'-dihdyroxy-3,3,3',3'-tetramethyl-1,1'-spiro(bis)indane, hereinafter designated "SBI", have negative intrinsic birefringences, owing to the molecular structure of the SBI unit and conformation in said homopolymers.
A class of copolycarbonates having low intrinsic birefringence is disclosed, for example, in U.S. Pat. No. 4,552,949. Said copolycarbonates comprise structural units derived from bisphenol A and SBI.
It is also known, however, that SBI polycarbonates are deficient in such areas as processability and ductility, as demonstrated, for example, by very high glass transition temperatures (Tg). One result is that molding of SBI polycarbonates, including both homopolycarbonates and copolycarbonates also containing bisphenol A units, induces severe stresses. This is particularly true of injection molding of optical disks, in which such stresses are magnified. Under such conditions, these stresses can induce significant birefringence in a disk, despite the low intrinsic birefringence of the SBI-containing polycarbonates.
A critical parameter in the performance of an optical disk is termed "vertical" birefringence (hereinafter "VBR"), which is defined as the difference between the refractive indices for light polarized perpendicular to the plane and that polarized in the plane of the disk. High VBR is a problem often encountered in disks molded from SBI polycarbonates in addition to the other processing difficulties. The physical properties accompanying such processing difficulties may include glass transition temperatures above 200.degree. C. and high melt viscosities.
The vertical birefringence parameter is characteristic of optical disks specifically and cannot be related directly to polycarbonates in bulk. However, a related parameter which may be determined for a bulk polycarbonate is its stress optical coefficient in the glass phase, Cg. The Cg value for bisphenol A polycarbonate is 72, while that of a bisphenol A-SBI copolycarbonate containing 72 mole percent SBI units is 26.
It is of interest, therefore, to develop polycarbonates having both low Cg values and high processability and ductility.